Sense of Calm Is Swept Away in the Final Minutes

At 3:50 p.m., the Dow Jones industrial average was nearly flat for the day, an encouraging end to a volatile session and, perhaps, a sign that some manner of relief had started to come to Wall Street.

Ten minutes later, the Dow was down 203.18 points. The Standard & Poor’s 500-stock index had dropped 3.2 percent. Stocks that had soared earlier were flashing red. And investors were left to ponder the remains of another lost day.

“That was a little weird,” Sam Stovall, chief investment strategist at S.& P., said, moments after the market skidded at 20 points a minute. “Usually, you get a sense of it in the last hour or half-hour, not the last 5 or 10 minutes.”

But, he added, stocks may now be in a permanent state of anything goes.

“In some ways you can say, ‘Are you really surprised?’ ” Mr. Stovall said. “It’s been engaging in this kind of slithering pattern for a while.”